1. Field of the Invention
This invention relates to a driving circuit for a brushless motor having a permanent-magnetic rotor, and more particularly to a driving circuit for a motor for driving a fan (hereinafter referred to as "fan motor").
2. Description of the Prior Art
In general, a driving circuit for a fan motor is adapted to detect the position of magnetic poles of a magnetic rotor using a rotor position detecting means to supply exciting current to exciting windings of the stator in turn depending upon an output signal of the rotor position detecting means.
However, the failure of rotation of the rotor due to heavy load or the like causes exciting current to continue to flow through only one of the exciting windings, often resulting in the overheat of the exciting winding or in the worst case the burning. To prevent such defect, it is required to interrupt the exciting current when it continues to flow through the exciting winding over a predetermined period of time.
For this purpose, a detecting circuit has been proposed which serves to count time during which a rotor is stopped and generate a failure signal indicating that an excess of the counted time over a predetermined period of time (set time) means the occurrence of failure of the rotor, and used in a manner to allow the supply of exciting current to be stopped when the failure signal is generated from the detecting circuit.
However, irrespective of the use of such detecting circuit, another defect still cannot be eliminated that the use of a power supply having characteristics that the rising time of voltage of the power supply is lengthened causes the starting time of the rotor or time required for the rotor to start to exceed the set time of the detecting circuit, so that the supply of exciting current is obliged to be stopped prior to the starting of the rotor to render the starting substantially impossible. It is assumed that the elimination of such defect is carried out by lengthening the set time of the detecting circuit. Unfortunately, this causes a significant disadvantage that lengthening of the set time renders the prompt stop of supply of exciting current substantially impossible when the rotor is stopped due to heavy load or the like after the starting